The present disclosure relates to exhaust gas-driven turbochargers, and particularly to bypass arrangements that allow exhaust gas to bypass the turbine under certain engine operating conditions.
In order for an internal combustion engine to meet emission limits set by regulatory authorities, it is generally necessary to employ a catalyst system to reduce certain emission components to acceptable levels. The conversion efficiency of a catalyst is quite low when the catalyst is below a certain temperature, generally known as the “light-off” temperature. In a cold start of the engine, the catalyst is well below the light-off temperature, and hence the catalyst system has little effectiveness at reducing emissions. Accordingly, there is a need to hasten the heating of the catalyst as much as possible in order to minimize the amount of undesirable exhaust components emitted prior to light-off.
The cold-start problem is exacerbated by the presence of a turbocharger. In a conventional turbocharged engine system in which exhaust gases must pass through the turbine before reaching the catalyst, the turbocharger represents a heat sink for the exhaust stream. While turbochargers typically include a bypass valve or waste gate that can be activated to route the majority of the exhaust gas stream to the catalyst without having to pass through the turbine, conventional bypass valves or waste gates still allow a considerable amount of exhaust gas to pass through the turbine. This exhaust gas is cooled by the turbine and therefore slows the heating of the catalyst, delaying the attainment of light-off temperature.